Update load.php with wp_is_bad_request function

I propose adding a wp_is_bad_request function to load.php, as a way to allow plugin developers to short circuit code execution if the HTTP request that started the WordPress load process is considered a junk request. See: #17246

The existing wp_favicon_request (renamed to wp_favicon_request_x in test file) is re-written to use the proposed wp_is_bad_request function.